The use of anti-theft devices employing an access key with electronic identification is becoming more and more diffused. Nowadays such a device is present as standard equipment in practically all new cars. Commonly these access keys with electronic identification contain in the handle portion thereof a contactless powering device gathering electromagnetic energy from the electronic circuitry installed in the lock into which the key is inserted and communicating its identification code to the circuitry of remote contactless powering and recognition of the code installed in the lock. If the powering and code recognition device installed in the lock fails to detect the valid code, it blocks the lock and in case of car does not enable the starting of the engine.
These devices are particularly effective because the mere duplication of the key is useless if its identification code is unknown.
The access key may even be in the form of a programmed smart-card or a common key containing in its handle portion a transmitter.
According to a common technique, the electronic circuits contained in coded key are remotely powered without establishing any electric contact with an electric source such as a battery by exploiting magnetic induction. In practice, the remote powering and code recognition system installed in the lock upon inserting the key, generates an electromagnetic field forcing an AC current of a relatively high frequency, usually in the order of kHz, in a (primary) winding.
The circuit encased in the key comprises a (secondary) winding coupled to dedicated power supply pins of an integrated circuit (microchip). When the key is introduced in the lock, the (secondary) winding contained in it is immersed in the electromagnetic field generated by the (primary) winding of the lock system for contactlessly powering the circuits contained in the key and for eventually detecting and recognizing its code.
Substantially, the windings behave as the primary and secondary windings of a transformer in air. On the (secondary) winding present in the key an electromotive force, which is commonly rectified and regulated, for supplying the functional circuits contained in the integrated circuit. The so powered functional circuits produce a certain coded identification signal.
A very effective way of producing a detectable signal corresponding to the identification code of the key is to make the functional circuits of the integrated circuit contained in the key, that constitute an electric load of the secondary winding of the air-core transformer, absorb energy in a discontinuous or variable manner according to a certain temporal pattern. In this way a certain time-based pattern of variation of the amplitude of the voltage in the primary winding of the contactless powering device of recognition and validation of the identification code of the key is induced.
A comparator of the circuitry of recognition of the device detects the time-base pattern of current absorption variations, generating a corresponding logic signal in the form of a sequence of bits of the key identification code, that is inviolably stored in a nonvolatile manner in the integrated circuit contained in the key.
A diagram of a common car anti-theft device using the described system is depicted in FIG. 1.
When the key is inserted in the lock 5, a certain voltage is induced on the secondary winding 3, which may even be integrated on the microchip, contained in the handle of the key 4 together with an integrated circuit nonvolatily storing a certain digital identification code. Therefore, the microchip, depicted as a gray shaded rectangle, is powered with this voltage.
Once the functional integrated circuits contained in the microchip are so powered, the secondary circuit of the air transformer will alternate phases in which it absorbs a certain current to phases in which the current absorption becomes almost negligible.
Therefore, a variable current (in practice a discontinuous current) will circulate in the secondary winding 3 of the air transformer of the contactless powering system and thus, because of mutual induction, a back electromotive force will induced on the primary winding 2 of the device PR installed in the lock 5.
This back electromotive force reduces the amplitude of the alternated current present on the nodes of the primary winding, as shown in FIG. 2a. 
A common comparator contained in the device PR installed in the lock 5, detects these amplitude variations of the alternated voltage on the primary winding 2 generating an active logic signal when the amplitude is smaller than a pre-established threshold.
This logic signal, shown in FIG. 2b, is sampled at a pre-established clock frequency for providing the bit sequence of the identification code of the key in the lock. If the microprocessor contained in the device PR installed in the lock 5 does not recognize (validate) the code, it inhibits ignition of the motor 1.
The whole process of contactless powering and recognition of the code takes a relatively short time, typically about 100 ms.
Unfortunately such a system is completely useless in case of thieving of the key or robbery because the lawful owner cannot prevent the use of the stolen key by a thief.